is similar to re_match (see GNU Matching) except that you
pass two data strings and sizes, and an index stop beyond
which you don’t want the matcher to try matching. As with
re_match, if it succeeds, re_match_2 returns how many
characters of string it matched. Regard string1 and
string2 as concatenated when you set the arguments start and
stop and use the contents of regs; re_match_2 never
returns a value larger than size1 + size2.